Memory provides data storage for electronic systems. Flash memory is one type of memory, and has numerous uses in modern computers and devices. For instance, modern personal computers may have BIOS stored on a flash memory chip. As another example, it is becoming increasingly common for computers and other devices to utilize flash memory in solid state drives to replace conventional hard drives. As yet another example, flash memory is popular in wireless electronic devices because it enables manufacturers to support new communication protocols as they become standardized, and to provide the ability to remotely upgrade the devices for enhanced features.
NAND may be a basic architecture of integrated flash memory. A NAND memory cell may include a charge-storage material between a conductive gate and a channel region. The charge-storage material may correspond to a floating gate (e.g., may comprise polycrystalline silicon or other suitable semiconductor), or to a charge-trapping material (e.g., silicon nitride, silicon oxynitride, conductive nanodots, etc.).
In some applications a NAND structure may comprise charge-trapping material (for instance, silicon nitride) spaced from a channel region by a barrier dielectric material. In operation, the NAND structure has two distinguishable memory states based upon whether or not a sufficient number of electrons are trapped by the charge-trapping material. Programming of the NAND structure may comprise migration of electrons through the barrier dielectric material and onto the charge-trapping material. Erasure of the NAND structure may comprise detrapping of electrons and migration of the detrapped electrons back through the barrier dielectric material.
A problem that occurs is that it can be difficult to remove the trapped electrons, and accordingly unreasonably high voltages may be required for erasure of NAND structures. An approach toward solving such problem is to provide a thin layer of silicon nitride within the barrier dielectric. The thin layer of nitride may enable hole injection to assist in the detrapping (erase) operation. However, the thin layer of nitride within the barrier dielectric may undesirably provide parasitic trapping which pulls trapped electrons from the primary charge-trapping material of the NAND structure. The parasitic trapping may lead to decay of a memory state, and accordingly to undesired volatility of data stored within the NAND structure.
It is desired to develop improved NAND structures which overcome the problems and difficulties described above.